BERLIN — German Chancellor Angela Merkel's would-be successor said on Monday she will not run for the premiership, propelling the country into uncertainty as the tenure of its longtime leader nears an end.

The success of Merkel protegee Annegret Kramp-Karrenbauer in a party leadership contest last year had been seen as validation for the centrist chancellor, who has served for 15 years and said she won't run for reelection in 2021.

But Kramp-Karrenbauer, also known as AKK, struggled to assert her authority as head of Merkel's Christian Democrats, culminating in a political crisis last week, when party representatives in the eastern region of Thuringia defied her instructions and aligned with the far-right.

She announced Monday that she would step down as party leader when a new candidate for the premiership is chosen. That leaves Merkel's legacy unsecured. It also opens the party and the country to leadership by political figures who want to steer it further to the right.

While no candidates have officially announced, the presumed front-runners include two figures who challenged Kramp-Karrenbauer for the leadership post last time. Friedrich Merz — who last week said he would quit his board position at BlackRock to focus on politics — and Health Minister Jens Spahn both say the party needs to take a hard line on immigration and generally track right in the face of declining support for centrists and the rise of the far-right Alternative for Germany party (AfD).

Armin Laschet, the moderate leader of North Rhine-Westphalia, Germany’s most populous state, is also considered a potential candidate and would be more in the Merkel mold.

“We are currently feeling strong centrifugal forces within our society and party,” Kramp-Karrenbauer said in a news conference Monday. “We have to be stronger, stronger than today.”

She suggested that the separation of the role of chancellor and the party leadership — with Merkel staying on as premier while ceding the party chairmanship — had weakened the Christian Democrats. Kramp-Karrenbauer said she didn’t believe that should happen again.

“This was the very first time in party history that the chancellor remained but the party leadership changed,” said Michael Meister, a Christian Democrat parliamentarian, who added that the result had been “difficult.”

Merkel moved fairly late to cultivate a successor, and when she did, she chose someone who had yet to be proven on a national stage. Kramp-Karrenbauer was leader of Saarland, Germany’s second smallest state, before Merkel nominated her as general secretary of the Christian Democratic Union in February 2018, opened the position of party leader with her in mind that December and tapped her as German defense secretary last July.

“Merkel was very successful in biting off challengers,” said Arndt Leininger, a political scientist with the Free University of Berlin. “But she pretty much failed in terms of a successor.”

He said there had been “high hopes” for Kramp-Karrenbauer initially, but she quickly became embroiled in controversy, denounced for insulting transgender people with a joke about gender-neutral bathrooms.

The position of German defense minister has long been seen as a poisoned chalice, with postwar Germany maintaining a reluctant attitude toward militarism and the armed forces accused of inefficiency. In that job, Kramp-Karrenbauer came under criticism for announcing support for a safe zone in Syria without consultation.

Her greater problems, though, came in her role as party leader. And last week’s controversy in Thuringia marked the end of her path to the chancellery.

She had instructed the local branch of her party to drop out of later rounds of voting for the state premiership to avoid any possibility of alignment with the AfD, according to Meister, the Christian Democrat lawmaker. But local representatives defied her guidance. The Christian Democrats and the AfD ended up backing the same candidate: Thomas Kemmerich of the Free Democrats.

The move was seen as a violation of a pledge by Germany’s mainstream parties not to cooperate with the far-right, as well as a break in the postwar political consensus, and it prompted street demonstrations in German cities.

Despite traveling to the state capital to meet regional party chiefs, Kramp-Karrenbauer failed to persuade them to hold a fresh round of elections.

Merkel, returning from a trip to South Africa, had to step in to contain the crisis. She fired a minister for the region who had applauded the election, worked to avert a possible breakup of her governing coalition and pressured Kemmerich to resign as state premier, which he did.

The episode highlighted the challenge facing Germany’s mainstream parties as they navigate a fractured political environment where coalitions are the norm while vowing that they won’t partner with either the far-right or the left-wing Die Linke.

Meister said his party needed an answer to the issue, “but the answer can only be to show that we are a democratic alternative.”

Cooperating with parties that do not seek democracy is a red line, he said. “We saw what happened in Germany in the 1930s,” he said.

Kramp-Karrenbauer announced her planned resignation in a meeting with politicians from her party on Monday morning.

“It was a shock,” said Elmar Brok, a veteran politician who was present. He said that the party faces a “difficult” future but that a new clear leadership process could also bring opportunity.

Kramp-Karrenbauer said she will remain defense minister, a decision that government spokesman Steffen Seibert said Merkel supports “wholeheartedly.”

However, whether Germany’s ruling coalition will last and Merkel will hold on until her term is up remain open questions.

luisa.beck@washpost.com